In small bowl, mix powdered sugar and cocoa. Fold cocoa mixture, 1/3 at a time, into beaten egg whites. Spoon mixture into decorating bag fitted with star tip. Draw 1 1/2-inch diameter circle on white paper; place under parchment paper as a guide for piping rounds. Pipe mixture into twenty-four 1 1/2-inch rounds on parchment-lined cookie sheet. With back of teaspoon, make an indentation in each to hold filling. (Or, mixture can be spooned into dollops on parchment paper.)

Meanwhile, in 1-quart heavy saucepan, heat whipping cream just to simmering over medium-low heat. Remove from heat; stir in chocolate with wire whisk until melted. Stir in butter pieces, a few at a time, until melted. Refrigerate until thickened, about 30 minutes.

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Just before serving, spoon or pipe about 1 teaspoon filling into indentation of each meringue.

Expert Tips

Cooking parchment paper is a grease- and moisture-resistant paper that prevents baked goods from sticking to pans. It also keeps the pans clean. Look for it near the foil at the grocery store.

Cookies can be made up to 24 hours before serving; store in tightly covered container. Up to 2 hours before serving, spoon in filling and store in refrigerator until ready to serve.